(1) Field of the Invention:
This invention relates to a photonic device such as a light-emitting device and a photodetector, and a substrate for fabricating such a photonic device, in which multilayered thin films made of III-V nitride semiconductor are epitaxially grown.
(2) Background of the Invention
A III-V nitride semiconductor material is commercially used for a photonic device such as a light-emitting device and a photodetector. As the above III-V nitride semiconductor material, an AlxGayInzN (x+y+z=1, x, y, zxe2x89xa70) material is widely available, that is, each layer of the photonic device is composed of the AlxGayInzN film that is epitaxially grown by a MOCVD method. In this basic case, TMA (trimethyl aluminum) is employed as the Al raw material and TMG (trimethyl gallium) is employed as the Ga law material, and TMI (trimethyl indium) is employed as the In law material. Moreover, NH3 is employed as the nitrogen raw material. N2 gas and/or H2 gas is used as a carrier gas.
Then, various control of the above raw materials in flow rate can change the composition of the AlxGayInzN film. An AlN film has its bandgap Eg of 6.2 eV, and a GaN film has its bandgap Eg of 3.4 eV. Therefore, in the case of forming an AlxGal-xN film using TMA and TMG, the AlxGal-xN film has substantially its bandgap of 6.2x+3.4(1xe2x88x92x)eV, and has substantially its emission wavelength xcex=1240/{6.2x+3.4(1xe2x88x92x)} from an equation xcex=1240/Eg. Given x=0.3, the emission wavelength xcex is 292 nm. In this case, the detection wavelength is below 292 nm.
In the case of fabricating a light-emitting diode from the AlxGayInzN (x+y+z=1, x, y, zxe2x89xa70) multilayered thin films, when the AlxGayInzN film is epitaxially grown on a C-faced sapphire substrate by a MOCVD method, it includes large amount of defect, resulting in the deterioration of its crystallinity and thus the deterioration of its light-emitting efficiency.
From this point of view, it is proposed that the AlxGayInzN (x+y+z=1, x, y, zxe2x89xa70) multilayered thin films is formed on the sapphire substrate via a buffer layer made of a GaN film which is epitaxially grown by a CVD at low temperature. The GaN buffer layer supplements the lattice constant of 10% and over between the sapphire substrate and the multilayered thin films, and provides a favorable crystallinity to the multilayered thin films. Instead of the GaN buffer layer, an AlN buffer layer may be employed.
A conventional light-emitting device as mentioned above can emit a light having only 400 nm or over. Therefore, the AlxGayInzN multilayered thin films are required to have a relatively large amount of the Al component in order to emit a short wavelength blue light or a short wavelength ultraviolet light. Moreover, for emitting a green to blue light, all the AlxGayInzN films except a light-emitting layer are required to have a relatively large amount of the Al components, respectively, in order to confine energy in the light-emitting layer effectively. However, if the Al-rich AlxGayInzN tin film is formed on the buffer layer, made of e.g. the GaN film or the AlN film, epitaxially grown by the CVD at low temperature, it may bring about cracks in the Al-rich AlxGayInzN thin film and deteriorate the crystallinity thereof.
The reason is that since the Al-rich AlxGayInzN thin film has a smaller lattice constant, a large tensile stress may be brought about in the thin film due to the large difference in the lattice constants between the thin film and the buffer layer if the thin film is formed on the buffer layer. Moreover, the lateral growth speed of the Al-rich AlxGayInzN thin film is very small, and thus, the enhancement of the crystallinity of the thin film is hindered by the poor crystalline buffer layer due to the low temperature epitaxial growth. Moreover, in a photodetector such as a UV photodetector, its detecting sensitivity is degraded due to the poor crystallinity of the buffer layer.
In order to iron out the above matters, such a light-emitting device having a Al-rich AlxGayInzN multilayered thin films on a buffer layer made of an AlxGal-xN (1xe2x89xa7x greater than 0) film is disclosed and proposed in the publication of unexamined patent application, Tokukai Hei 9-64477 (JP A 9-64477).
Moreover, such a light-emitting device is disclosed in the publication of unexamined patent application, Tokukai Hei 5-291628 that plural Gal-x-yInxAlyN (1xe2x89xa7xxe2x89xa70, 1xe2x89xa7xxe2x89xa70) thin films having their various x- and/or y-components are formed on a sapphire substrate to obtain a predetermined Gal-a-bInaAlbN (1xe2x89xa7axe2x89xa70, 1xe2x89xa7bxe2x89xa70) buffer layer, and then, the Gal-a-bInaAlbN (1xe2x89xa7axe2x89xa70, 1xe2x89xa7bxe2x89xa70) multilayered thin films are formed on the buffer layer.
In Tokukai Hei 9-64477, since the AlGaN buffer layer is formed at a relatively high temperature, the Al-rich AlxGayInzN multilayered thin films epitaxially grown on the buffer layer can have relatively favorable crystallinity and does not have cracks therein.
However, the AlGaN buffer layer requires to be formed at a high temperature of 1300xc2x0 C. or over, and annealed at a high temperature of about 1500xc2x0 C. after the formation of the buffer layer. Such a high temperature treatment overloads a heater in a MOCVD apparatus, resulting in the complication of the maintenance and the increase of the manufacturing cost.
Particularly, in realizing a photonic device to emit or detect an above-mentioned short wavelength light, since the required Al-rich AlxGayInzN multilayered thin films has its respective small vertical and lateral growth speed, its high film-forming temperature must be held for a long time, thus overloading a heater and so on of a MOCVD apparatus.
When the above AlGaN buffer layer having a thickness of 0.3 xcexcm is formed at about 1200xc2x0 C., many cracks come into being in the buffer layer and the crystallinity of the buffer layer is deteriorated. As a result, the entire crystallinity of the Al-rich AlxGayInzN multilayered thin films is deteriorated.
In Tokukai Hei 5-291618, since the Gal-a-bInaAlbN (1xe2x89xa7axe2x89xa70, 1xe2x89xa7bxe2x89xa70) multilayered thin films is formed on the Gal-a-bInaAlbN (1xe2x89xa7axe2x89xa70, 1xe2x89xa7bxe2x89xa70) buffer layer composed of the laminated plural Gal -x-yInxAlyN (1xe2x89xa7xxe2x89xa70, 1xe2x89xa7xxe2x89xa70) thin films having their various x- and/or y-components, it can have its favorable crystallinity and almost never have cracks therein. Moreover, since the buffer layer is formed at a low temperature of about 700xc2x0 C., a heater in a MOCVD apparatus is not overloaded.
However, in the above conventional fabricating method, the multilayered thin films and the buffer layer have the same component and composition, so that they are continued though their boundaries. In this case, a leak current is flown to the buffer layer from the multilayered thin films, resulting in the reduction of the light-emitting efficiency of the light-emitting device having the multilayered thin films due to the resistance loss.
It is an object of the present invention to provide a photonic device having an AlaGabIncN (a+b+c=1, a, b, cxe2x89xa70) buffer layer and an AlxGayInzN (x+y+z=1, x, y, zxe2x89xa70) multilayered thin films without cracks epitaxially grown on the buffer layer which have their favorable crystallinities, and a substrate for fabricating the photonic device.
It is another object of the present invention to provide a method for fabricating the photonic device and a method for manufacturing the photonic device-fabricating substrate.
For achieving the above objects, this invention relates to a photonic device comprising a substrate, a buffer layer with a composition of AlaGabIncN a+b+c=1, a, b, cxe2x89xa70) formed on the substrate and a multilayered thin films with a composition of AlxGaylnzN (x+y+z=1, x, y, zxe2x89xa70) epitaxially grown on the buffer layer, the Al component of the Al component-minimum portion of the buffer layer being set to be larger than that of at least the thickest layer of the multilayered thin films, the Al component of the buffer layer being decreased continuously or stepwise from the side of the substrate to the side of the multilayered thin films therein.
Moreover, this invention also relates to a substrate for fabricating a photonic device comprising a substrate, a buffer layer with a composition of AlaGabIncN a+b+c=1, a, b, cxe2x89xa70) formed on the substrate, the Al component of the Al component-minimum portion of the buffer layer being set to be larger than that of at least the thickest layer of the multilayered thin films to constitute the photonic device, the Al component of the buffer layer being decreased continuously or stepwise from the side of the substrate to the side of the multilayered thin films therein.
In the present invention, the Al component of the Al component-minimum portion of the buffer layer is set to be larger than that of at least the thickest layer of the multilayered thin films. This requisite is for preventing cracks in the multilayered thin films. The largest tensile stress may occur in the thickest layer of the multilayered thin films, so that cracks may be also likely to occur in the thickest layer. Therefore, a compressive stress is brought about according to the above requisite for preventing the occurrence of the cracks.
Moreover, in the present invention, the Al component of the buffer layer is decreased continuously or stepwise from the side of the substrate to the side of the multilayered thin films.
If the buffer layer is formed at a low temperature of about 1200xc2x0 C., it requires to be formed thick, e.g. 1 xcexcm to 2 xcexcm to obtain its good crystallinity. However, such a thick buffer layer has a large tensile stress therein because the lattice constant of the buffer layer is increased, so that cracks may come into being in the buffer layer.
FIGS. 1 and 2 are graphs explaining the above phenomenon. In FIG. 1, the horizontal axis designates the thickness of the AlN portion adjacent to the substrate of the buffer layer, and the longitudinal axis designates the FWHM of X-ray rocking curve at (002) peak of the AlN portion to evaluate the crystallinity of the AlN portion. As is apparent from FIG. 1, as the thickness of the AlN portion is increased, the FWHM is decreased and thus, the crystallinity of the AlN portion is be developed.
In FIGS. 2A and 2B, the horizontal axis designates the thickness of the AlN portion of the buffer layer, and the longitudinal axes designate the lattice constants xe2x80x9caxe2x80x9d and xe2x80x9ccxe2x80x9d of the bottom surface of the hexagonal columnar crystal of the AlN portion shown in FIG. 3, respectively. The heavy line designates ideal lattice constants xe2x80x9caxe2x80x9d and xe2x80x9ccxe2x80x9d of an AlN film having a hexagonal columnar crystal, respectively. As is apparent from FIG. 2, as the thickness of the AlN portion is increased, the lattice constant xe2x80x9caxe2x80x9d is elongated and the lattice constant xe2x80x9cbxe2x80x9d is shrunk.
Therefore, as the thickness of the AlN portion adjacent to the substrate of the buffer layer is increased, the crystallinity of the AlN portion is enhanced and the bottom surface lattice constant xe2x80x9caxe2x80x9d is increased. As a result, as the thickness of the buffer layer made of the AlaGabIncN (a+b+c=1, x, y, zxe2x89xa70) is increased, a tensile stress occur in the buffer layer horizontally and thus, cracks are likely to come into being.
In order to prevent the cracks, it is considered that the upper side of the buffer layer is made of a large lattice constant material before the some cracks come into being in the buffer layer. As the Al component of the AlaGabIncN (a+b+=1, a, b, cxe2x89xa70) is decreased, the lattice constant thereof is increased. Therefore, according to the present invention, if the Al component of the AlaGabIncN constituting the buffer layer is decreased continuously or stepwise from the side of the substrate to the side of the multilayered thin film to constitute the photonic device, the lattice constant of the upper side of the buffer layer is increased, so that the tensile stress does not occur and thus, the cracks are prevented even though the buffer is formed thick at a low temperature of about 1200xc2x0 C.
Moreover, if the buffer layer includes the Ga component from the AlaGabIncN (a+b+c=1, a, b, cxe2x89xa70), the lateral growth speed of the buffer layer can be developed, so that the amount of dislocation in the buffer layer can be reduced.
In a preferred embodiment of the photonic device according to the present invention, the Al component-maximum layer of the multilayered thin films is made of AlxGayInzN (x+y+z=1, 1.0xe2x89xa7Xxe2x89xa70.3) suitable for a short wavelength light-emitting device or a short wavelength photodetector.
In another preferred embodiment of the present invention, the Al component-minimum portion is made of AlaGabIncN (a+b+c=1, 1.0xe2x89xa7axe2x89xa70.5), preferably AlaGabIncN (a+b+c=1, 1.0xe2x89xa7axe2x89xa70.7).
In still another preferred embodiment of the present invention, the adjacent portion to the substrate of the buffer layer has a composition of AlN. In this case, since the buffer layer can have a large degree of freedom for the Al component therein, the above requirement of the present invention can be easily satisfied. As a result, a photonic device having the multilayered thin films without cracks having their favorable crystallinities can be provided efficiently.
In a further preferred embodiment of the present invention, an interface to divide the buffer layer upward and downward by 10 atomic % or more of Al component is formed in the buffer layer. Particularly, in the photonic device of the present invention, it is desired that an interface to divide the buffer layer and the multilayered thin films by 10 atomic % or more of Al component is formed. If the buffer layer or the photonic device has such a large Al component step therein, dislocations can not travel upward through the large Al component step. As a result, dislocations in the upper side from the above interface, that is, the large Al-component step, can be reduced and thus, the crystallinity of the multilayered thin films can be enhanced.
Furthermore, this invention relates to a method for fabricating a photonic device comprising the steps of preparing a substrate, forming a buffer layer with a composition of AlaGabIncN (a+b+c=1, a, b, cxe2x89xa70) by a MOCVD method, and epitaxially growing a multilayered thin films with a composition of AlxGayInzN (x+y+z=1, x, y, zxe2x89xa70) by a MOCVD method, on condition that the Al component of the Al component-minimum portion of the buffer layer is set to be larger than that of at least the thickest layer of the multilayered thin films, and the Al component of the buffer layer is decreased continuously or stepwise from the side of the substrate to the side of the multilayered thin films therein.
Moreover, this invention relates to a method for manufacturing a photonic device-fabricating substrate comprising the steps of preparing a substrate, and forming a buffer layer with a composition of AlaGabIncN (a+b+c=1, a, b, cxe2x89xa70) by a MOCVD method, on condition that the Al component of the Al component-minimum portion of the buffer layer is set to be larger than that of at least the thickest layer of the multilayered thin films to constitute the photonic device, and the Al component of the buffer layer being decreased continuously or stepwise from the side of the substrate to the side of the multilayered thin films therein.
In a preferred embodiment of the present invention related to the method for fabricating a photonic device and the method for manufacturing a photonic device-fabricating substrate, the buffer layer is formed at a higher film-forming temperature than that of the multilayered thin films to constitute the photonic device. Thereby, the crystallinity of the buffer layer including a larger amount of Al component than that of the multilayered thin films can be enhanced effectively.
Concretely, the buffer layer is formed at 1100xc2x0 C. or over. Moreover, the buffer layer is preferably formed at a temperature less than 1300xc2x0 C., as mentioned above, for mitigating load to some components such as a heater in a MOCVD apparatus. From this point of view, the buffer layer of the present invention may be called as a xe2x80x9chigh temperature buffer layerxe2x80x9d, compared with the above conventional buffer layer epitaxially grown at a low temperature of about 700xc2x0 C.
Moreover, it is desired that carrier gas flow rate ratio (H2 carrier gas flow rate/N2 carrier gas flow rate) in forming the buffer layer is set to be larger than the one in forming the multilayered thin films.
Furthermore, it is desired that raw material gas flow rate ratio (V raw material gas flow rate/III raw material gas flow rate) in forming the buffer layer is set to be larger than the one in forming the multilayered thin films. Herein, the flow rate of the III raw material gas is calculated from its saturated vapor pressure, provided that the III raw material gas is not polymerized, for example, dimerized.
In addition, in using a III raw material gas including Al component, an average gas flow rate including the above raw material gases and the above carrier gas above the substrate in the reactor of a MOCVD apparatus is preferably set to be 1 m/sec or over. The average gas flow rate is obtained from the following equation (1):
xe2x80x83{Summation of gas flow rates converted at 0xc2x0 C. (L/min)/60xc3x97103xc3x97cross sectional area above the substrate in the reactor of a MOCVD apparatus (m2)}xc3x97{760/pressure inside the reactor (Torr)}xe2x80x83xe2x80x83(1)
That is, as the summation of the gas flow rates is increased, and/or the cross sectional area of the reactor is decreased, and/or the pressure inside the reactor is decreased, the average gas flow rate is increased. When the average gas flow rate is set to be 1 m/sec or over, the raw material gases do not almost react one another in a vapor phase in the reactor of the MOCVD apparatus, so that the crystallinity of the buffer layer can be developed effectively.
Moreover, in the method for fabricating a photonic device and the method for manufacturing a photonic device-fabricating substrate of the present invention, preferably, an interface to divide the buffer layer upward and downward by 10 atomic % or more of Al component is formed in the buffer layer. Particularly, in the method for fabricating a photonic device of the present invention, preferably, an interface to divide the buffer layer and the multilayered thin films by 10 atomic % or more of Al component is formed. Thereby, the crystallinity of the multilayered thin films can be developed through the reduction of dislocations in the multilayered thin films.